It is known that materials having a nano-size structure exhibit characteristics which are different from those of a bulk state. Among these materials, a nanofiber having a nanometer thickness and a length, which is several tens times greater than the thickness, exerts the size effect peculiar to a fiber shape because of its high aspect ratio, and therefore an intense interest has been shown towards the nanofiber as one of advanced materials. A silica nanofiber has a high aspect ratio and a large surface area peculiar to the nanofiber and also has various physical properties such as semiconductor characteristics, conductivity, surface physical properties and mechanical strength peculiar to an inorganic material, and therefore it is expected to be widely applied in various fields of advanced materials including electronic materials and biology & life sciences. It is also expected that applications of the silica nanofiber are remarkably increased by assembling a nanofiber (one-dimensional) thereby forming a fabric-shaped (two-dimensional) or massive (three-dimensional) structure as a structure while maintaining characteristics of the nanofiber.
Particularly, materials obtained by combining a silica nanofiber with the other functional material such as inorganic material or organic material have wide applicability and it is expected that materials obtained by combining the silica nanofiber with the inorganic material such as metals are widely applied in various fields of electronic materials, optical materials, catalysts, coloring materials and sensors.
As materials obtained by combining silica with metals or metal ions, for example, a composite material produced by fixing a metal complex to a mesoporous silica is used in chemical reaction catalysts, electrochemical sensors, and solid polymer electrolytes. When a complex produced by introducing a metal complex into a mesoporous silica is applied, since various merits such as high surface area of the silica surface, uniform distribution of a complex active site in a nanocavity, rapid diffusion of a substrate compound, and heat resistance and acid resistance of a catalyst carrier are expected, an intense interest has been shown towards a metal complex fixation technique in which a mesoporous silica is used as a carrier (see Non-patent reference 1).
However, the silica used as a conventional composite material of the metal complex and the silica was limited to a bulk powder or a particle state of the silica. Therefore, since the complex fine particles have a particle shape in which an aspect ratio is about 1:1, only the complex fine particles cannot be assembled or integrated, and thus it was difficult to form a structure in which properties peculiar to a nano structure material are maintained.
Also, the production method requires the step of introducing an amino or imino group for coordinate bonding of metal ions into silica backbones through a chemical bond, and the step was complicated.
Various studies have been made on the fine composite material of the silica and the metal, for example, a complex of mesoporous silica/metal nanowire produced by reducing a metal ion solution in a channel of a mesoporous silica of MCM-41 series (see Non-patent reference 2, and Non-patent reference 3) and a complex of silica fine particles/metal nanoparticles produced by bonding metal ions in silica fine particles (see Non-patent reference 4).
However, in a complex of a conventional metal nanoparticle/nanowire and the other material, it is difficult to form a metal nanoparticle or a nanowire unless the fixed space of a material having a fixed shape. Therefore, the shape of the silica has low degree of freedom, and the silica is limited to a bulky silica having pores which form a wire or silica having a particle shape, and it was difficult to control the shape of the complex. Consequently, it was difficult to highly integrate the complex or the metal nanowire included therein.
[Non-patent reference 1]
B. Lee et al., Langmuir, (2003), 19, p 4246-4252
[Non-patent reference 2]
G. Hornyak et al., Chem. Eur. J. 1997, 3, No. 12, p 1951-1956
[Non-patent reference 3]
Yong-Jin Han, Chem. Mater., 2000, 12, p 2068-2069
[Non-patent reference 4]
V. G. Pol et al., Chem. Mater., 2003, 15, p 1111-1118